Topics, Tools and Techniques in Paleoclimate Research

Stable isotope geochemistry: Organics
Speaker: Steve Leavitt,  LTRR
February 18th 2004
 
Summary provided by Peggy Barker

Dr. Leavitt started his talk by asking the class to name the different proxies mentioned in the Baker paper. After the list was compiled, of which there were nine proxies, he stated that all the proxies mentioned involve plant activity. From there Dr. Leavitt launched into a talk about terrestrial carbon.

Dr. Leavitt focused on the isotopic composition of organic materials, specifically plants. To look at the isotopic composition of plants, the plant's components need to be looked at separately, since the isotopic composition of plants is component dependent. He explained that carbohydrates are the least robust in plants, and that cellulose is more robust, making cellulose the compound of choice to study. Studies have been done looking at cellulose versus whole wood as a function of climate. Similar trends can be seen for both analyses. Can only whole wood be looked at, simplifying things? Problems with this idea were raised.

False latewood bands were also discussed. They have been studied and used as markers, and are an indication of aridity. δ¹³C, for the portion of the tree ring between the latewood and the false latewood, was measured and plotted against precipitation, giving a hint of some link between moisture and δ¹³C.  Mike adds: the carbon isotopic composition of plant matter is determined by the balance between fractionation during photosynthesis and the equilibration of intraleaf carbon dioxide with atmospheric carbon dioxide concentrations via leaf openings or stomata.  Since the extent to which the plant opens or closes its stomata also affects leaf water content, δ¹³C can be used as a proxy for atmospheric moisture or water use efficiency in plants.

Next, Dr. Leavitt talked about C3 and C4 plants and their distribution. Mike adds: since C3 and C4 plants have mean carbon isotopic compositions of -27 and -12 permil PDB, respectively, variations in theδ¹³C of soil organic material over time can be used to determine the  variations in overlying vegetation type.  Through soil analyses from various points around the Midwest, a vegetation history was developed, showing the northward migration of C4 plants over the past 13,000 years.  

We were also told how δ¹³C in bones can be useful in several ways. An analysis to determine the δ¹³C in bones found in burials sites from the Fremont Culture in Utah was performed.  This was a useful method for anthropological research, and for climate studies.

Finally, Dr. Leavitt touched upon δ¹³C in pollen analysis, and how it’s possible to see variations in the same species due to climate.




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